Bioacoustics refers to the creation and reception of sound waves among biological organisms. Measured sound emissions by plants as well as differential germination rates, growth rates and behavioral modifications in response to sound are well documented. Plants detect neighbors by means other than well-established communicative signals including volatile chemicals, light detection, direct contact and root signaling. Because sound waves travel efficiently through soil and can be produced with minimal energy expenditure, plants may use sound as a means for interpreting their environment and surroundings. Preliminary evidence supports that plants create sound in root tips when cell walls break. Because plant roots respond only to sound waves at frequencies which match waves emitted by the plants themselves, it is likely that plants can receive and transduce sound vibrations into signals to elicit behavioral modifications as a form of below ground communication.
Buzz pollination, or sonication, serves as an example of a behavioral response to specific frequencies of vibrations in plants. Some 2000 plants species, including Dodecatheon and Heliamphora have evolved buzz pollination in which they release pollen from anthers only when vibrated at a certain frequency created exclusively by bee flight muscles. The vibrations cause pollen granules to gain kinetic energy and escape from pores in the anthers.
Plants emit audio acoustic emissions between 10–240 Hz as well as ultrasonic acoustic emissions (UAE) within 20–300 kHz. Evidence for plant mechanosensory abilities are shown when roots are subjected to unidirectional 220 Hz sound and subsequently grow in the direction of the vibration source. Using electrograph vibrational detection, structured sound wave emissions were detected along the elongation zone of root tips of corn plants in the form of loud and frequent clicks. When plants are isolated from contact, chemical, and light signal exchange with neighboring plants they are still able to sense their neighbors and detect relatives through alternative mechanisms, among which sound vibrations could play an important role. Furthermore, ultrasonic acoustic emissions (UAE) have been detected in a range of different plants which result from collapsing water columns under high tension. UAE studies show different frequencies of sound emissions based on whether or not drought conditions are present. Whether or not UAE are used by plants as a communication mechanism is not known.
Although the explicit mechanisms through which sound emissions are created and detected in plants are not known, there are theories which shed light on possible mechanisms. Mechanical vibrations caused by charged cell membranes and walls is a leading hypothesis for acoustic emission generation. Myosins and other mechanochemical enzymes which use chemical energy in the form of ATP to produce mechanical vibrations in cells may also contribute to sound wave generation in plant cells. These mechanisms may lead to overall nanomechanical oscillations of cytoskeletal components, which can generate both low and high frequency vibrations.